1. Field of the Invention
The present invention relates to metal-free phthalocyanine having novel features, a process for preparing the same, and a high-sensitivity electrophotographic photoconductor using such metal-free phthalocyanine as a charge-generating substance and having a high sensitivity particularly to a light in the near infrared region such as a semiconductor laser radiation. The present invention further relates to novel processes for respectively preparing .alpha.-type metal-free phthalocyanine or X-type metal-free phthalocyanine.
2. Related Art
Electrophotography is an image-forming method wherein photoconductivity is skillfully combined with an electrostatic phenomenon as disclosed in U.S. Pat. No. 2,297,691 to Carlson. According to this method, the surface of a photoconductor, it's electrical conductivity increases at receiving light, is uniformly charged in the dark by corona discharge or the like, then irradiated with a light image from a manuscript or the like to turn the light image into an electrostatic latent image through utilization of the photoconductivity of the photoconductor, and then has a charged colored powder (toner) attached thereto to turn the latent image into a visible image of the toner, which is then transferred to a support such as paper and then fixed thereon to obtain an image thereon.
According to the foregoing electrophotography, digital processing of an image is possible when a manuscript image is exposed to and scanned with a light from a laser or an LED used as an exposure light source to divide the manuscript image into picture elements, pixels, and the surface of a photoconductor is then exposed to and scanned with a light while using the pixels as the units of the light. Further, printout of information is possible when the surface of a photoconductor is exposed to and scanned with light dots changed from the output of an information processing apparatus. In view of the foregoing, high-speed optical printers and digital copiers of high image quality utilizing electrophotography have recently rapidly come into wide use in keeping with the rapid advance and increasing speed of information processing. Particularly in the case of using a semiconductor laser as a light source, apparatuses of the kind as mentioned above can be miniaturized, decreased in cost, and increased in reliability and image quality. Thus, apparatuses using a semiconductor laser as the light source thereof have attracted attention. Accordingly, active research and development of a photoconductor having a sensitivity to a semiconductor laser radiation are under way. Since the oscillation wavelength of a semiconductor laser now in practical use is limited to comparatively long wavelengths in the near infrared region, a photoconductor for use in an apparatus using a semiconductor laser as the exposure light source thereof is required to have a high photosensitivity to a light having a long wavelength in the near infrared region. Further, such a photoconductor is required to be free of a fluctuation in electrical properties, a fluctuation in output images, and memorization even when used in a variety of environments as well as repeatedly used.
Conventional charge-generating substances known as those sensitive to the oscillation wavelength of a semiconductor laser include polyazo pigments, phthalocyanine pigments, azulenium salt pigments, pyrilium salt pigments, and naphthoquinone pigments. However, the naphthoquinone pigments are comparatively low in sensitivity, the polyazo pigments are difficult to synthesize stably, and the azulenium salt pigments and the pyrilium salt pigments are easy of a change in photoelectric properties in relation to humidity. Further, these pigments disadvantageously are so unstable in an aspect of chemical structure when exposed to a strong light such as a laser radiation that a difficulty has been experienced in putting them into practical use. On the other hand, the phthalocyanine pigments are chemically and physically stable pigments as have been used as coloring pigments in inks, paints, etc., and it has moreover been known since the discovery thereof that they have properties as organic semiconductors. It has further been known that the absorption wavelength and photoconductive properties of a phthalocyanine pigment are greatly varied depending on the kind of central metal thereof, the crystal form thereof and the method of treatment thereof. From these standpoints, the phthalocyanine pigments have widely been studied because the discovery of a phthalocyanine pigment having a high sensitivity to a light in the long wavelength region has been expected.
Metal phthalocyanines known as charge-generating substances include .epsilon.-type copper phthalocyanine, chloroaluminum phthalocyanine (AlPcCl), titanyl phthalocyanine (TiOPC), and chloroindium phthalocyanine (InPcCl). However, .epsilon.-type copper phthalocyanine, though stable as a substance, is unsatisfactory in sensitivity to the semiconductor laser radiation, while chloroaluminum phthalocyanine and chloroindium phthalocyanine each having a central metal bonded to a halogen atom disadvantageously are unstable to water and hence changeable in electrical properties. On the other hand, titanyl phthalocyanine, though excellent in sensitivity to a light in the long wavelength region, involves such problems that a difficulty is encountered not only in stably securing a crystal form excellent in properties but also in stably maintaining such a crystal from for a long period of time, and that titaninum oxide, which is an n-type semiconductor, formed as a by-product from titanium chloride or a titanium alkoxide as a starting material of synthesis of titanyl phtalocyanine is hardly removed therefrom.
On the other hand, since metal-free phthalocyanine is also a chemically and physically stable compound as can be understood from the use thereof as a pigment like copper phthalocyanine, it has been studied as a charge-generating substance like metal phthalocyanines. Known crystal forms of metal free phthalocyanine include thermodynamically unstable .alpha.-type and .beta.-type, thermodynamically stable .beta.-type, and a variety of other types. Since .alpha.-, .beta.- and .gamma.-type crystal forms of metal-free phthalocyanine are insufficient in sensitivity to a light in the near infrared region involving the oscillation wavelength of a semiconductor laser, research and development have been made with a view to investigating other crystal forms of metal-free phthalocyanine, examples of which include X-type metal-free phthalocyanine as disclosed in U.S. Pat. No. 3,357,989, .tau.-type metal-free phthalocyanine as disclosed Japanese Patent Application Laying-open No 182,639, .gamma.'-type metal-free phthalocyanine as disclosed in Japanese Patent Application Laying-open No. 87,332/1985, high-purity X-type metal-free phthalocyanine different in crystal habit as disclosed in Japanese Patent Application Laying-open No. 243,089/1985, metal-free phthalocyanine as disclosed in Japanese Patent Application Laying-open No. 47,054/1987, and metal-free phthalocyanine as disclosed in Japanese Patent Application Laying-open No. 233,769/1990.
In an aspect of properties, X-type metal-free phthalocyanine is one of representative crystal forms of metal-free phthalocyanine first expected to be applied to photoconductors for laser printers. U.S. Pat. No. 3,357,989 discloses a process for preparing such X-type metal-free phthalocyanine, wherein a commercially available starting material is dissolved in sulfuric acid and then precipitated in iced water to obtain purified .alpha.-type metal-free phthalocyanine, which is then milled. The above-mentioned U.S. Patent specification also discloses that as many hours as 48 to 168 hours of ball milling of the purified .alpha.-type metal-free phthalocyanine are required to obtain X-type metal-free phthalocyanine in the case of using a ball mill in the foregoing process (see "Comparative Example 4" in the U.S. Patent specification), while 2 hours of milling of the purified .alpha.-type metal-free phthalocyanine enable X-type metal-free phthalocyanine to be obtained in the case of using a speck mixer mill in the foregoing process. The speck mixer mill is however a small milling apparatus for a dentist, and the use thereof is therefore unsuitable in an industrial process. Since the process for preparing X-type metal-free phthalocyanine as disclosed in U.S. Pat. No. 3,357,989 is impractical, various processes for preparing X-type metal-free phthalocyanine have heretofore been studied. For example, Japanese Patent Application Publication No. 8,102/1970 discloses a process wherein .alpha.-type metal-free phthalocyanine is mixed with an organic aliphatic vehicle in the presence of a minor part of seed crystals of X-type metal-free phthalocyanine to be mostly converted into X-type metal-free phthalocyanine. According to this process, however, a difficulty is encountered in stably obtaining X-type crystal because .beta.-type crystals are obtained in many cases as demonstrated in Comparative Example 1 as will be described later. On the other hand, Japanese Patent Application Publication No. 42,511/1971 discloses a process wherein a phthalonitrile derivative is reacted in an ammonia-saturated alkylalkanolamine solvent in the presence of seed crystals of X-type metal-free phthalocyanine to synthesize X-type metal-free phthalocyanine. According to this process, however, the conversion into X-type crystal is only about 70% in Examples in the above-mentioned Japanese Patent Publication specification. As described hereinbefore, a difficulty has been experienced in easily and stably obtaining X-type metal-free phthalocyanine.
On the other hand, .tau.-type metal-free phthalocyanine is prepared by effecting wet milling of .alpha.-type metal-free phthalocyanine together with a grinding assistant such as sodium chloride and an inactive solvent such as ethylene glycol at a temperature of 50.degree. C. to 180.degree. C., preferably 50.degree. C. to 180.degree. C., for 5 hours to 20 hours, thus making the processing thereof complicated and difficult. Therefore, a difficulty is encountered in stably securing a constant crystal form of .tau.-type metal-free phthalocyanine. The preparation of .tau.-type metal-free phthalocyanine also involves similar disadvantages. As for the high-purity X-type metal-free phthalocyanine different in crystal habit as disclosed in Japanese Patent Application Laying-open No. 243,089/1985, removal of a strong base used as a catalyst is difficult to make the product unstable in properties, and the method of preparation thereof is very complicated and hence impractical, as described in Japanese Patent Application Laying-open No. 233,769/1990, the application of which was filed by the same applicant as in Japanese Patent Application Laying-open No. 243,089/1985. As for the metal-free phthalocyanine as disclosed in Japanese Patent Application Laying-open No. 47,054/1987, it is prepared by firstly preparing X-type metal-free phthalocyanine and then treating the same with a nonpolar organic solvent such as tetrahydrofuran, thus involving not only problems of long-time steps and a high production cost but also a demerit of an insufficient sensitivity of the product because of a large particle size thereof resulting from the growth of primary particles through the treatment thereof with the solvent. As for the metal-free phthalocyanine as disclosed in Japanese Patent Application Laying-open No. 233,769/1990, it is pointed out in Japanese Patent Application Laying-open No. 328,170/1992, application of which was filed by the same applicant as in Japanese Patent Application Laying-open No. 233,769/1990, that it has an insufficient sensitivity and involves a demerit that the residual potential thereof is greatly increased through repeated use thereof.
Thus, metal-free phthalocyanines known to date involve some demerits such as a difficulty in stable preparation thereof on an industrial scale, an insufficient sensitivity, and an instability in electrical properties thereof. Most of these defects are attributed to the influences of impurities in metal-free phthalocyanine. More specifically, the presence of a large amount of such impurities entails a problem that necessary crystal conversion is hard to effect stably and perfectly. Further, charge carriers generated upon reception of a light are entrapped in the impurities and imperfect portions of crystals to bring about a lowering of the sensitivity of metal-free phthalocyanine, a fluctuation in electrical properties thereof through repeated used thereof, and a delay in time constant, which are causative of memorization as well as a change in the density of an output image.
From these standpoints, methods of purification of metal-free phthalocyanine are under energetic study with an eye to improving the properties thereof as a charge-generating substance.
Most general methods of preparing metal-free phthalocyanine, as disclosed on page 153 in Moser and Moser "Phthalocyanine Compound" [ACS Monograph No. 157; Reinhold Publishing Corp., New York (1963)] and U.S. Pat. No. 3,357,989, are a so-called acid pasting method wherein pigment phthalocyanine is dissolved in sulfuric acid and the resulting solution is brought into contact with water to precipitate phthalocyanine again, and a sublimation method wherein pigment phthalocyanine is sublimed under reduced pressure. The sublimstion method is excellent in purification of phthalocyanine, but unsuitable for industrial production thereof because the throughput thereof cannot be increased. On the other hand, according to the acid pasting method, for example, side reactions such as decomposition of metal-free phthalocyanine with sulfuric acid occur, complete removal of sulfate ions is difficult, and impurities are left in and between crystal particles of phthalocyanine because of very rapid re-precipitation of dissolved phthalocyanine with water, thus involving a difficulty in sufficient purification of phthalocyanine.
As the method of avoiding a decomposition reaction with sulfuric acid as well as the method of avoiding residual sulfate ions, there can be mentioned methods wherein metal-free phthalocyanine is once converted into a solvent-soluble metal phthalocyanine or phthalocyanine complex and insoluble impurities in a solvent are then removed through filtration, followed by reversion thereof to the original metal-free phthalocyanine to thereby effect purification thereof. More specifically, according to a first method of the kind as described above, metal-free phthalocyanine is converted into dilithium phthalocyanine, which is then dissolved in an alcohol, and the resulting solution is filtered to remove therefrom impurities such as insoluble through filtration and then admixed with water, a dilute aqueous solution of a mineral acid, carbon dioxide gas, dry ice or the like to decompose the dilithium phthalocyanine into the original metal-free phthalocyanine. According to a second method of the kind as described above, metal-free phthalocyanine is converted into dipotassium phthalocyanine and formed into a complex with a solvent to effect solubilization thereof in a solvent, and then purified in substantially the same manner as in the first method to effect reversion of the complex to the original metal-free phthalocyanine. Known examples of such a complex include a dipotassium phthalocyanine-dicrown ether complex as disclosed in J. Am. Chem. Soc., 103, 4629 (1981); dipotassium phthalocyanine-DMSO complex as disclosed in U.S. Pat. No. 4,197,242; and dipotassium phthalocyanine-DMF complex as disclosed in Inorg. Chem. 20, 2709 (1981). Methods of preparing X-type metal-free phthalocyanine using metal-free phthalocyanine highly purified according to either of these methods are disclosed in Japanese Patent Application Laying-open No. 243,089/1985 and Japanese Patent Application Laying-open No. 115,085/1986. However, such solubilization-in-solvent methods involve a problem that they are very complicated in the production steps, and hence substantially impractical. Furthermore, dilithium phthalocyanine and dipotassium phthalocyanine complexes, though soluble in a solvent, are not so high in solubility that a large amount of the solvent must disadvantageously be used to increase the production cost. Moreover, impurities such as inorganic ions remain in the resulting product.
Meanwhile, the aforementioned crystal forms sensitive to a light in the near infrared region are all meta-stable crystal forms positioned between the .alpha.-type as a thermodynamically unstable crystal form and the .beta.-type as a thermodynamically stable crystal form. In general, crystal form conversion hardly proceeds from a thermodynamically stable crystal form into a thermodynamically unstable crystal form. The crystal conversion of metal-free phthalocyanine from the .beta.-type as a stable crystal form hardly proceeds toward a meta-stable crystal form, which is a thermodynamically more unstable crystal form, and a considerable amount of .beta.-type crystals remains in the product even if it proceeds toward the more unstable crystal form, thus failing to provide good results for the product as a charge-generating substance. In view of the foregoing, synthesized and purified .beta.-type metal-free phthalocyanine is once converted into an unstable .alpha.-type or .gamma.-type crystal form, which is then converted into the desired semi-stable crystal form. Since such conversion of .beta.-type crystals into .alpha.-type or .gamma.-type crystals is usually effected according to the aforementioned acid pasting method as described in connection with purification of the phthalocyanine, however, there arises a problem that a decomposition by-product of metal-free phthalocyanine, sulfate ions, etc. remain in the product metal-free phthalocyanine. Other known methods include a method wherein phthalocyanine is sublimed in high vacuum at a low temperature, a method wherein .beta.-type crystal is converted into .alpha.-type crystal by a mechanical shear applied thereto with a ball mill or the like at a low temperature, and a method wherein phthalocyanine is synthesized in a lower alcohol at a low temperature of at most 70.degree. C. However, the sublimation method cannot be employed on an industrial scale, while the conversion method using the mechanical strain involves a problem that complete conversion into .alpha.-type crystal is difficult to leave a considerable amount of .beta.-type crystal, and the low-temperature synthesis method involves demerits such as a large amount of impurities remaining in the product, and a low yield. On the other hand, Japanese Patent Application Laying-open No. 115,085/1986 discloses in Example 3 that the layer of dipotassium phthalocyanine-didigylme complex is treated with a 5% aqueous solution of hydrochloric acid to obtain .alpha.-type metal-free phthalocyanine. However, this method involves substantially the same problem as described in connection with the above-mentioned acid pasting method.
As described hereinbefore, the organic charge-generating substances sensitive to a light with a wavelength in the near infrared region such as the semiconductor laser radiation include the phthalocyanine compounds. However, the conventional phthalocyanine compounds as have been known to date involve various problems with the purity thereof, the method of preparation thereof, and the processing thereof. Thus, no phthalocyanine compound well satisfactory in properties as a charge-generating substance has been obtained yet.